Lasers are the brightest and, in many instances, some of the most efficient sources of light available today. Nevertheless, there is a constant need to increase their brightness and total power even further. There are, however, real barriers to increasing the brightness of a single laser source of any design that currently present some of the most difficult technological challenges. One of the best examples is semiconductor lasers. Semiconductor lasers provide the highest electrical-to-optical power-conversion efficiency, and achieve very high optical gain, but typically do not provide sufficiently high contiguous gain volume. As a result, optical power in excess of 10 W (i.e., continuous wave) can currently be extracted only from arrays of semiconductor laser diodes. Such ensembles of individual laser emitters, however, cannot readily provide a diffraction-limited or nearly diffraction-limited beam, even though they can deliver very impressive total output power (e.g., ˜500 W/cm2 of array face area currently achieved). This results in poor brightness of otherwise highly efficient and powerful light sources.
The poor mode quality of laser diode arrays stems from the fact that the gain is extracted from physically separate pump regions, essentially making them extended area light sources. However, if there was a technique to harness the gain of such separate gain regions and make them behave as parts of a contiguous gain medium with a constant phase difference between all the regions, then the light emitted by all such regions would be coherent and, as a result, could be efficiently combined into a single beam of much higher optical quality than that of the standard laser diode array. This problem is common to all laser types and, if solved, would allow for harnessing together the power of multiple laser sources with progressively increasing brightness, without having to deal with the problems of scaling up the power of each individual source.
Other methods of beam combination for increased brightness include polarization combining and spectral beam combining. Polarization beam combining, however, can only increase brightness by a factor of two. Spectral beam combining, on the other hand, does not increase spectral brightness.